Channel induction furnaces are used in industries for melting a metal (which term includes metal alloys) and maintaining the metal in a molten state. For example, channel induction furnaces are used in galvanising and foundry industries for melting Zn-containing alloys and Al-containing alloys, including Al/Zn-containing alloys, and maintaining the alloys in a molten state.
A known channel induction furnace comprises (a) a steel shell, (b) a lining of a refractory material, such as an aluminosilicate, internally of the shell, (c) a pot for containing a bath of molten metal that is defined by the refractory-lined shell, and (d) one or more than one channel inductor for heating metal that is connected to the shell and in fluid communication with the pot via a throat that extends through the refractory-lined shell to an inlet in the channel inductor.
The channel inductor comprises (i) a steel shell, (ii) a lining of a refractory material, such as an aluminosilicate, (iii) a channel defined by the refractory-lined shell that forms a path for molten metal to flow from the pot through the channel and back into the pot, and (iv) an electromagnetic coil which generates an electromagnetic field. At any given time during the operation of a channel induction furnace, molten metal in the channel of the channel inductor becomes a secondary circuit of a transformer and is heated and kept molten by currents induced by the electromagnetic field. The channel inductor is a bolt-on assembly on the shell of a channel induction furnace. The refractory material that so forms the lining is selected to accommodate a range of specific mechanical requirements, thermal insulation requirements, and resistance to chemical attack by molten metal. These requirements are competing requirements to a certain extent in the sense of needing different material properties and hence the selection of the refractory material tends to be a compromise.
Channel inductors have a limited life when exposed to molten metals such as Zn-containing and Al-containing alloys and typically fail in the following modes:                Cracking of the refractory material, particularly along central planes of channel inductors, during heat-up, dry-out, or operation, and subsequent penetration of Zn and/or Al metal or Zn vapours into the cracks which extend the cracks, ultimately resulting in a metal leak from the channel inductors.        Additionally, in the case of Al-containing alloys, by reduction of SiO2 in the refractory material by Al, thereby forming Al2O3 and Si, with an associated reduction in the volume of the refractory material and penetration and/or spalling of the refractory material.        Additionally, blocking due to adherence of corundum growth within the channel, which is compounded by pieces of altered refractory or dross from a pre-melt pot main area entering the channel.        
Typically, the life of channel inductors in Al-containing alloys is 6-24 months and is one of the main reasons for metal coating line shut-downs.
The applicant is developing a new inductor having greater reliability and, more particularly, less tendency so to fail due to cracking.
The new inductor is described in International publication WO2011/120079 in the name of the applicant.
The channel inductor that is described and claimed in the International publication comprises (a) a channel liner that is formed from a refractory material that is resistant to chemical attack by the molten metal in the channel and is the only material of the channel inductor that is in direct contact with the molten metal and (b) a back-up liner that supports the channel liner and is formed from a refractory material that is optimal for thermal insulation material properties and mechanical strength properties, such that the integrity of the channel liner and is not compromised during heat-up, dry-out, or operation of the channel induction furnace.
A channel inductor made with a channel liner and a back-up liner in accordance with the invention of the International publication was found to have issues with cracking when used on a manufacturing plant of the applicant for coating steel strip with Zincalume® molten metal.
The applicant has investigated the causes of the cracks and the present invention was made in the investigation.
The above discussion is not intended to be a statement of the common general knowledge in Australia and elsewhere.